1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus for optically recording information on an optical disk or optical memory and optically reproducing the information therefrom.
2. Description of the Related Art
In a optical information recording/reproducing apparatus, a semiconductor laser is used as a light source for generating a light beam. In the type of optical information recording/reproducing apparatus, the light beam is converged onto an optical disk or an optical memory by an objective lens. The light beam reflected from or optical disk is returned to the semiconductor laser through the objective lens. If the light beam returns to the semiconductor laser, the optical intensity of the light beam emitted from the semiconductor laser may be adversely affected, with the result that information cannot be reliably recorded or reproduced from the optical disk. In particular, in an opto magnetic recording/reproducing apparatus wherein information is recorded on a magneto-optical recording medium (e.g., a magneto-optical disk), a change which the light beam may have in a plane of polarization is detected, so that the light beam is not completely prevented from returning to the semiconductor layer. More specifically, when the magneto-optical recording/reproducing apparatus is in the recording mode, a recording light beam is irradiated onto the recording surface or layer of the optical disk. With the direction of magnetization of the recording layer being changed by the recording light beam, information is recorded on the disk. When the magneto-optical recording/reproducing apparatus is in the reproduction mode, the direction of polarization of the light beam reflected by the recording layer is changed in accordance with the direction of magnetization of the recording surface, so as to reproduce the information from the disk. That is, the plane of polarization of the light beam reflected by the recording layer is modulated in accordance with the information. In the detection optical system of the magnet-optical recording/reproducing apparatus, therefore, a change which the recording surface has in the direction of polarization is detected. In the magnet-optica recording/reproducing apparatus, an optical element, such as a beam splitter, is arranged between the semiconductor laser and the optical disk, so as to separate the light beam returning from the magneto-optical disk from the light beam directed toward the magneto-optical disk. However, since the optical element cannot completely separate the two light beams, part of the light beam directed to the magneto-optical disk undesirably returns to the semiconductor laser. In order to reliably operate the semiconductor laser, the conventional optical recording/reproducing apparatus separates part of the light beam emitted from the semiconductor laser to the optical disk from the remaining parts and causes the separated part of the light beam to be detected by a photodetector. In accordance with a detection signal from the photodetector, the driving current to be supplied to the semiconductor laser is controlled in such an magnet-optical manner as to stabilize the optical intensity of the light beam emitted from the semiconductor laser. Examples of such a light beam-detection optical system are shown in FIGS. 1 and 2. In the detection optical system shown in FIG. 1, a beam splitter 18 separates part of the light beam emitted from a semiconductor laser 1 from the remaining parts, and the separated part of the laser beam is detected by a photodetector 8 without being processed. Such a detection system is disclosed in Published Unexamined Japanese Patent Application No. 63-13143. In the detection system shown in FIG. 2, a beam splitter 18 separates part of the light beam emitted from a semiconductor laser 1 from the remaining parts, and the separated part of the laser beam is detected by a photodetector 8 after being condensed by a convergent lens 19.
Problems mentioned below are pointed out with respect to the conventional detection systems mentioned above. With respect to the system which does not incorporate a convergent lens, it is difficult to provide the light-receiving surface with an appropriate area. If this area is too large, a large amount of detection beam will be received by the light-receiving surface, but the junction capacitance of the photodetector will be large. If the area is too small, a sufficient amount of detection beam will not be received by the light-receiving surface. In either case, the controllable range of the driving current to be supplied to the semiconductor laser cannot be widened, so that the optical intensity of the light beam emitted from the semiconductor laser cannot be stabilized reliably. With respect to the system which incorporates a convergent lens, the light-receiving efficiency of the photodetector may be improved, but the size of the optical system is undesirably increased in accordance with the focal length of the convergent lens.
As mentioned above, the conventional optical information recording/reproducing apparatus has problems in that the light-receiving surface of the photodetector, which receives a monitor light beam, must have a large area to improve a light receiving efficiency. If the light-receiving surface of the photodetector have an magnet-optical large area, the control band width of the system is narrowed. In the case where the conventional optical information recording/reproducing apparatus incorporates a convergent lens so as to improve the light-receiving efficiency of the photodetector, the size of the optical system is increased in accordance with the focal length of the convergent lens.